Human acceptance of renal and liver transplants after withdrawal of all immunosuppression has recently been found to be associated with an active form of peripheral immune regulation, alternatively termed "bystander" or "donor antigen-linked" suppression. Bystander suppression, which can be detected by means of a human-to-mouse adoptive transfer ('trans-vivo') assay, has the following features: 1) Peripheral blood mononucleocyte (PBMC)-mediated delayed type hypersensitivity (DTH) responses are a) weak/absent to donor cells, soluble alloantigens and allopeptides, b) normal/strong to recall antigens alone, c) normal/strong to recall antigens when third party or self antigens are present and d) weak/absent to recall antigens when donor alloantigens are present; 2) The failure to respond to donor antigen and the failure to respond to recall antigen in the presence of donor antigen can be reversed by the inclusion of antibodies to tumor growth factor (TGF)beta or interleukin (IL)-10 or both at the DTH challenge site; and 3) A single donor antigen or peptide is sufficient to trigger DTH inhibition. We propose to define the key components of this process, including: 1) the various donor soluble antigens which drive regulation versus effector responses; 2) microchimerism as a possible source of both soluble antigens and direct pathway (membrane-bound) antigen presentation in peripheral lymphoid tissue and host monocyte-lineage antigen-presenting cells (APC) essential for the DTH response; and 3) host alloantigen-specific CD4+ and CD8+ T cells which produce, or cause to be produced, the regulatory cytokines IL-10 and TGFbeta. Our source of PBMC for these studies will be human allograft "acceptors" - patients who have ceased all immunosuppression yet retained graft function. We hypothesize that chronic stimulation by low levels of soluble antigens and rare donor-derived leukocytes drives development of two distinct populations of host T lymphocytes: 1) antigen-specific regulatory T cells and 2) antigen- specific DTH effector T cells whose activity is masked by regulatory T cell- dependent TGFbeta and IL-10 release. Our goal is to develop a working model of how donor-derived leukocytes and the antigens they release contribute to a tonic equilibrium between host T regulator and effector cells in human allograft acceptance.